Currently, you will have to cancel the existing dialog and then supply your own. With version 2.0.20072.229 (available off our prerelease site here) you can cancel the default appointment form and handle your own dialog on the client side.
I put together a sample that demonstrates how you can make a custom dialog using C1WebDialog and C1WebDateEdit. This allows you change how the dialog will look and only include fields you want, a basis for any further customization. The sample also demonstrates how you can add, update and delete appointments from the client side (javascript).
Regards,
Greg L